In recent years, liquid crystal display devices are used not only in the conventional application to personal computer monitors but also in application to ordinary color televisions. The color reproduction range of the color liquid crystal display devices is determined by colors of light emitted from the red, green and blue pixels and, where chromaticity points of the respective color pixels in the CIE XYZ colorimetric system are represented by (xR,yR), (xG,yG), and (xB,yB), the color reproduction range is represented by an area of a triangle defined by these three points on an x-y chromaticity diagram. Namely, the larger the area of this triangle, the more vivid color image the display devices reproduce. The area of this triangle is normally expressed by a ratio of the area of the triangle to an area of a reference triangle formed by three points of the three primary colors, red (0.67, 0.33), green (0.21, 0.71) and blue (0.14, 0.08), in the standard system defined by U.S. National Television System Committee (NTSC) (in unit of %, which will be referred to hereinafter as “NTSC ratio”). The ordinary notebook computers have the values of approximately 40 to 50%, the desktop computer monitors the values of 50 to 60%, and the existing liquid crystal TVs the values of approximately 70%.
A color image display device utilizing such a color liquid crystal display device is constituted mainly by light shutters utilizing liquid crystal, a color filter having red, green and blue pixels, and a backlight for transmissive illumination, and the colors of lights emitted from the red, green and blue pixels are determined by the emission wavelength of the backlight and the spectroscopic curve of the color filter.
In the color liquid crystal display devices, the color filter extracts only wavelengths in necessary regions from the emission distribution of the backlight, to provide the red, green and blue pixels.
Methods for production of this color filter proposed heretofore include such methods as dyeing, pigment dispersion, electrodeposition, printing, ink jetting and so on. The colorants for coloring used to be dyes, but are now pigments in view of reliability and durability as liquid crystal display devices. Accordingly, at present, the pigment dispersion is most commonly used as a method for production of the color filter from the viewpoint of productivity and performance. In general, when the same colorants are used, the NTSC ratio and the brightness are in a trade-off relation, and the colorants are suitably selected for use depending on the particular application. Namely, if it is attempted to increase the NTSC ratio by adjusting the color filter in order to reproduce a vivid color image, the screen tends to be dark. Inversely, if the brightness is increased, the NTSC ratio tends to be low, and it tends to be difficult to reproduce a vivid image.
On the other hand, as a backlight, it has been common to employ one using as a light source a cold-cathode tube with emission wavelengths in the red, green and blue wavelength regions and using an optical waveguide plate for converting light emitted from this cold-cathode tube, to a white surface light source. In recent years, a light emitting diode (LED) has been used, since it has a longer operating life, requires no inverter, presents high brightness, is mercury free, etc.
Here, in a backlight employing conventional LED, blue light-emitting LED is used in such a manner that part of light emitted from this LED is converted to yellow light by a yellow-emitting phosphor, and white light obtained by color mixing of the blue light and the yellow light is used as a surface light source by means of an optical waveguide.
However, in the above light source, the yellow emitting phosphor was used, whereby emission with wavelengths unnecessary from the viewpoint of the color purity of red and green was substantial, and it was difficult to obtain a display with high color reproducibility (High Gamut). Here, it is possible, at least in principle, to increase the color purity of red and green by cutting off light with unnecessary wavelengths by means of a color filter, but as mentioned above, if it is attempted to increase the NTSC ratio by adjusting the color filter in order to reproduce the vivid color image, the majority of emission of the backlight will be cut off, whereby there has been a problem that the brightness decreases substantially. Especially, by this method, emission of red decreases substantially, whereby it has been practically impossible to reproduce a strongly reddish color.
In order to overcome this problem, a method of combining red-, green- and blue-emitting LEDs (Non-Patent Document 1) has been proposed, and by this method, a display having an extremely high color reproducibility has been prepared on a trial basis. However, in such a color image display device, LED chips independent for red, green and blue, respectively, are combined, whereby there have been problems such that 1) it takes time and labor to mount them, 2) since the respective LED chips for red, green and blue are disposed at finite distances, it is required to take the distance of an optical waveguide to be long to sufficiently mix emitted lights from the respective LED chips, and 3) since the white chromaticity is adjusted by combining the integral multiple of the respective LED chips, adjustment of the white balance can not be continuously carried out.
Further, Patent Document 1 discloses a color image display device having an NTSC ratio of at least 60%, which is constituted by a combination of a blue or deep blue emitting LED and a phosphor. With this color image display device, a broad color reproducibility may be attained as compared with the above mentioned yellow emitting phosphor, but emitted lights with wavelengths which are unnecessary from the viewpoint of the color purity of red and green, are substantial, and a still broader color reproducibility has been desired.
Further, Patent Documents 2 and 3 disclose semiconductor light emitting devices having specific phosphors combined, which is useful, for example, as a light source for backlight for e.g. liquid crystal displays. However, in a case where such semiconductor light emitting devices are practically combined with color filters to constitute color image display devices for e.g. liquid crystal displays, there have been cases where the emission of backlight tends to be inadequate, or chromaticity deviation is likely to occur.
On the other hand, WO2004/25359 (Patent Document 4) discloses that an image display device having a high NTSC ratio is obtainable by a combination of a light source for backlight satisfying specific conditions with a color filter. However, with respect to the demand for high performance in recent years, the specifically disclosed light emitting device employing the 3.5MgO.0.5MgF2.GeO2:Mn4+ type, Y2O3:Eu type and YVO4:Eu3+ type phosphors, is inadequate from the viewpoint of the emission efficiency, etc., and it is desired to develop one having higher performance.
Further, in recent years, a light emitting device employing K2TiF6:Mn has been known as a red emitting phosphor (Patent Documents 5 to 7). However, by a study by the present inventors, it has been found that such a light emitting device undergoes substantial deterioration of the properties and is not practically durable for a reason assumed to be such that hydrogen fluoride is generated by the reaction with moisture in air, and it has been desired to develop one having a higher performance.
Further, with a view to suppressing deterioration of phosphors, it is known to cover the surface of water-soluble phosphor particles with a coating of e.g. a metal oxide (Patent Document 8), but such a method requires a special apparatus and is inadequate from the viewpoint of the types of the useful phosphors, etc.    Non-Patent Document 1: Monthly display, April 2003 issue (p 42-46)    Patent Document 1: WO2005/111707    Patent Document 2: JP-A-2002-171000    Patent Document 3: U.S. Pat. No. 6,809,781    Patent Document 4: WO2004/025359    Patent Document 5: U.S. Patent Application Publication No. 2006/0071589    Patent Document 6: U.S. Patent Application Publication No. US 2006/0169998    Patent Document 7: U.S. Patent Application Publication No. US 2007/0205712    Patent Document 8: JP-A-2005-82788